In many positioning and driving systems for rotating mass memory devices, such as, for example, in hard disk (HD) and floppy disk (FD) drives, the L-R actuator, in these cases the motor, requires a peak current to speed up to a high speed during a first phase of the routine to find the right track. Such a driving system also typically requires a relatively low and controlled current during a successive tracking phase for precisely maintaining the position during the reading or the writing of data from and to the recordable media.
A typical driving system for a rotating mass memory media (hard disk drives, floppy disk drives, etc.) is depicted in FIG. 1. This figure shows how the driving current of a so-called voice coil motor (VCM) used in an HD drive is relatively high during a first phase A when the motor is forced to reach a relatively high speed in the shortest possible time. The figure also shows the driving current during phase B, when a reverse current, also relatively high, flows into the motor to initiate a braking action.
In a successive phase C, the current drops progressively when the motor approaches and reaches a constant speed. This permits a precise tracking during which the reading or writing operations take place. The last part of phase C is often referred to as the tracking phase when the write and read operations occur. In contrast, phases A and B and the first part (approach) of phase C define the so-called seeking phase, during which a high dissipation occurs.
The general requirement of reducing current consumption imposes the use of power devices with a low conduction resistance. This, according to state of the art integration techniques, implies the use of n-channel DMOS transistors, and the implementation of a pulse width modulation (PWM) driving of a full bridge power stage. The full bridge power stage is typically realized with four of the n-channel DMOS transistors.
Nevertheless, in many applications such as the one mentioned above where there is a tracking phase requiring a relatively low current though controlled with a high precision, the PWM driving cannot ensure an equivalent precision as that obtainable with a linear driving with operational amplifiers. On the other hand, the linear driving of an L-R actuator, for example a VCM motor, using a pair of operational amplifiers functioning in phase opposition according to a scheme as shown in FIG. 2 ensures a high precision, but is unfortunately penalized in terms of an increased power dissipation.
Approaches have been proposed for improving the performance of the driving system during the seeking and tracking phases to reduce consumption. To overcome the drawbacks of a linear mode driving system though preserving an acceptable tracking precision, a system has been proposed that would automatically switch from a PWM mode, employed during a first search phase to a linear mode used during the tracking phase, according to the scheme illustrated in FIG. 3. The two half-bridges that comprise the full bridge stage are driven by respective operational amplifiers. The inputs of the amplifiers receive a PWM signal generated by the respective converter or the error signal produced by the respective error amplifier of the driving system.
The disadvantage of such a system is the limited bandwidth when functioning in the linear mode, because of the presence of the power transistors (Md1, Md2, Md3 and Md4) that form the full bridge stage. The transistor dimensions and resulting electrical parameters are commensurate to support the relatively high current levels as required during the seeking phase. This limited bandwidth of the full bridge stage tends to excessively reduce the bandwidth of a linear control loop of the VCM motor during the critical tracking phase.
To appreciate the limitations and drawbacks of the above described system, it will be helpful to consider the response characteristic of the open loop system in a linear functioning mode and its peculiarities. FIG. 4 shows the open loop response of a driving system of a VCM motor, such as the one depicted in FIGS. 2 and 3 (in the latter case when functioning in a linear mode). The characteristic reproduced in FIG. 4 highlights the following aspects:
the presence of a dominant pole (Pd) that is inversely proportional to the product of the compensation capacitance (Ce), and the gain of the error operational amplifer and by the feedback resistance (Rf) of the current sensing operational amplifer; PA1 the presence of a second pole (P2) due to the inductance of the motor which tends to make the loop unstable; and PA1 the presence of a zero (Z) introduced to compensate the second pole, by dimensioning the capacitance (Ce) and resistance (Re) values of the compensation network of the error amplifier.
For instance, in a typical VCM motor of a hard disk drive, the pole introduced by the motor may be about 1 Khz. By dimensioning the values of the components of the compensation network of the error operational amplifer, the compensation zero may be fixed to a position just below the frequency of the motor pole. The value of the compensation capacitor may be established so as to fix the dominant pole of the loop to guarantee a phase margin of about 60.degree.. The gain bandwidth product (GBWP) of the error operational amplifier is the determinant factor that most influences the GBWP of the whole regulation loop.
To avoid a limitation of the regulation loop bandwidth, it is necessary to use driving operational amplifers, in cascade with the error amplifier, with a GBWP product of at least two orders of magnitude above the GBWP target value of the whole regulation loop. In other words, to obtain a sufficiently large bandwidth of the regulation loop, the bandwidth of the driving operational amplifers must be larger than that of the error amplifier. Therefore, the transfer function of the error amplifier may be multiplied by the gain without an attendant reduction of the phase margin. Indeed, to ensure a regulation loop GBWP of about 30 Khz, driving operational amplifers with a GBWP of about 3 Mhz, are needed. However, having to drive relatively large transistors with a small Rdson having relatively large intrinsic capacitances, the driving operational amplifers must be adequately compensated, and this significantly limits their GBWP.